Wireless or mobile (cellular) communications networks in which a mobile device (also referred to as User Equipment, UE, such as a mobile handset) communicates via a radio link to a network of base stations or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations. The initial deployment of systems using analogue signalling has been superseded by Second Generation (2G) digital systems such as Global System for Mobile communications (GSM), which typically use a radio access technology known as GSM Enhanced Data rates for GSM Evolution Radio Access Network (GERAN), combined with an improved core network.
Second generation systems have themselves been largely replaced by or augmented by Third Generation (3G) digital systems such as the Universal Mobile Telecommunications System (UMTS), which uses a Universal Terrestrial Radio Access Network (UTRAN) radio access technology and a similar core network to GSM. UMTS is specified in standards produced by 3GPP. Third generation standards provide for a greater throughput of data than is provided by second generation systems. This trend is continued with the move towards Fourth Generation (4G) systems.
3GPP design, specify and standardise technologies for mobile wireless communications networks. Specifically, 3GPP produces a series of Technical Reports (TR) and Technical Specifications (TS) that define 3GPP technologies. The focus of 3GPP is currently the specification of standards beyond 3G, and in particular an Evolved Packet System (EPS) offering enhancements over 3G networks, including higher data rates. The set of specifications for the EPS comprises two work items: Systems Architecture Evolution (SAE, concerning the core network) and Long-Term Evolution (LTE concerning the air interface). LTE uses an improved radio access technology known as Evolved UTRAN (E-UTRAN), which offers potentially greater capacity and additional features compared with previous standards. SAE provides an improved core network technology referred to as the Evolved Packet Core (EPC, also referred to as the Core Network, CN). Despite LTE strictly referring only to the air interface, LTE is commonly used to refer to the whole of the EPS, including by 3GPP themselves. LTE is used in this sense in the remainder of this specification, including when referring to LTE enhancements, such as LTE Advanced. LTE is an evolution of UMTS and shares certain high level components and protocols with UMTS. LTE Advanced offers still higher data rates compared to LTE and is defined by 3GPP standards releases from 3GPP Release 10 onwards. LTE Advanced is considered to be a 4G mobile communication system by the International Telecommunication Union (ITU).
Particular embodiments of the present invention may be implemented within an LTE mobile network (though the present invention may be considered to be applicable to many types of wireless communication network). Therefore, an overview of an LTE network is shown in FIG. 1. The LTE system comprises three high level components: at least one UE 102, the E-UTRAN 104 and the EPC 106. The EPC 106 communicates with Packet Data Networks (PDNs) and servers 108 in the outside world. FIG. 1 shows the key component parts of the EPC 106. It will be appreciated that FIG. 1 is a simplification and a typical implementation of LTE will include further components. In FIG. 1 interfaces between different parts of the LTE system are shown. The double ended arrow indicates the air interface between the UE 102 and the E-UTRAN 104. For the remaining interfaces user data is represented by solid lines and signalling is represented by dashed lines.
The E-UTRAN 104 comprises a single type of component: an eNB (E-UTRAN Node B, also referred to as a base station) which is responsible for handling radio communications between the UE 102 and the EPC 106 across the air interface. An eNB controls UEs 102 in one or more cell. LTE is a cellular system in which the eNBs provide coverage over one or more cells. Typically there is a plurality of eNBs within an LTE system. In general, a UE in LTE communicates with one eNB through one cell at a time.
Key components of the EPC 106 are shown in FIG. 1. It will be appreciated that in an LTE network there may be more than one of each component according to the number of UEs 102, the geographical area of the network and the volume of data to be transported across the network. Data traffic is passed between each eNB and a corresponding Serving Gateway (S-GW) 110 which routes data between the eNB and a PDN Gateway (P-GW) 112. The P-GW 112 is responsible for connecting a UE to one or more servers or PDNs 108 in the outside world. The Mobility Management Entity (MME) 114 controls the high-level operation of the UE 102 through signalling messages exchanged with the UE 102 through the E-UTRAN 104. Each UE is registered with a single MME. There is no direct signalling pathway between the MME 114 and the UE 102 (communication with the UE 102 being across the air interface via the E-UTRAN 104). Signalling messages between the MME 114 and the UE 102 comprise EPS Session Management (ESM) protocol messages controlling the flow of data from the UE to the outside world and EPS Mobility Management (EMM) protocol messages controlling the rerouting of signalling and data flows when the UE 102 moves between eNBs within the E-UTRAN. The MME 114 exchanges signalling traffic with the S-GW 110 to assist with routing data traffic. The MME 114 also communicates with a Home Subscriber Server (HSS) 116 which stores information about users registered with the network.
It will be appreciated that there is a need for UEs to communicate their capabilities to the network in order for subsequent communication with the network to function correctly. Specifically, UEs must communicate their capabilities, and in particular their Radio Frequency, RF, capabilities, to the eNB. One particular type of capability information is known as the UE E-UTRA capabilities (also referred to as LTE capabilities—both terms are used synonymously in this document) which must be reported to the network and stored by the network to facilitate future communication across the air interface (referred to as the Uu interface) between the UE and the eNB. The UE E-UTRA capabilities are reported in a UECapabilityInformation message which may be transmitted at any time to the network—specifically the eNB—in response to a UECapabilityEnquiry message. This may include, but is not limited to, the transmission of the UE E-UTRA capabilities during a network attach procedure.
The UE E-UTRA capabilities form part of the Radio Resource Control (RRC) protocol specification as defined by 3GPP TS 36.331 v12.6.0 released on 8 Jul. 2015 and available from http://www.3gpp.org/dynareport/36331.htm. The RRC protocol specifies, amongst other things, functions and parameters of the radio interface between the UE and the E-UTRAN. In particular, section 6.3.6 specifies certain Information Elements (IEs) including the UE E-UTRA capability IE which is used to convey E-UTRA UE Radio Access Capability Parameters, according to 3GPP TS 36.306, and Feature Group Indicators for mandatory features that the UE supports (defined in Annexes B.1 and C.1) to the network. The IE UE-EUTRA-Capability is transferred from the UE to the network in E-UTRA (that is when the UE is connected to an LTE network) or when the UE is connected to networks operating according to other Radio Access Technologies (RATs). 3GPP TS 36.306 according to v12.5.0 released on 7 Aug. 2015, and available from http://www.3gpp.org/dynareport/36306.htm, defines the E-UTRA UE radio access capabilities. As one example, section 4.3.4.7 of TS 36.306 specifies a supportedMIMOCapabilityDL-r10 field which defines the maximum number of spatial multiplexing layers in the downlink direction for a certain band and bandwidth class in a supportedBandCombination supported by the UE and is communicated as part of the UE E-UTRA capabilities IE.
It has been observed that the size of the UE E-UTRA capabilities, and hence the amount of data that must be communicated across the Uu interface between the UE and the eNB and stored by the eNB is rapidly growing. This is largely due to UEs increasingly supporting the aggregation of carriers (Carrier Aggregation, CA). That is, where UEs support more than one carrier in downlink or uplink (or both), by communicating with the E-UTRAN through multiple cells, the amount of UE E-UTRA capability information that is needed by the eNB is increased significantly. It has been identified that some further mechanism (or extension of existing mechanisms) needs to be introduced to constrain the volume of UE E-UTRA capability information.
One proposed partial solution to this problem is that the UE shall provide UE E-UTRA capabilities for CA where there are band combinations for 5 bands or more (beyond 5 carriers, b5c) only in response to a request from the E-UTRAN. That is, where the UE supports combinations of carriers which in combination total 5 downlink carriers or more (for instance, 6 carriers in downlink and 2 carriers in uplink) then the relevant UE E-UTRA capability information is not provided without request combinations for carriers beyond 5. However, in the event that the information is requested, a significant volume of data must still be transferred and stored. At best, the problem has been delayed, or avoided only in certain circumstances. Clearly communication between the UE and the eNB using 5 carriers or more is not possible until the eNB has obtained this information.
Additionally, a further factor that drives the increase in UE E-UTRA capability information is the reporting mechanism which is selected whenever a new optional feature is introduced that requires reporting. In particular when a new optional feature is introduced, which concerns the physical layer, a decision must be made regarding what kind of UE capability signalling (forming part of the UE E-UTRA capability information) should be introduced. The UE capability signalling may comprise one of the following options: a single bit (per UE) indicating whether the UE supports the new optional feature regardless of the UE's configuration; a bit per frequency band, allowing the UE to indicate that it supports the new optional feature for some frequency bands but not for others (for instance, half duplex, 256 QAM in download); a bit per frequency band combination; or a bit per frequency band within a band combination. To determine the appropriate option requires a trade-off between flexibility and optimal use of UE capabilities versus complexity and signalling overhead. Consequently, UE capability signalling for new optional features is evaluated on a case by case basis. Clearly, however, the desire to preserve maximum flexibility directly conflicts with efforts to constrain the growth of UE E-UTRA capability information.
It has also been proposed that fall back combinations of UE E-UTRA capabilities be specified, which concern less demanding configurations that a UE supporting a given configuration would also need to support: for instance with one component carrier less. Such a fall back configuration may not need to be signalled (within UE E-UTRA capabilities), unless the UE can support, for instance, more Multiple Input, Multiple Output (MIMO) layers for such a configuration.
Furthermore, UE E-UTRA capabilities may be uploaded when the UE is connected to a network operating according to another RAT. This is done principally so that the UE E-UTRA capability information can be provided to the eNB upon handover to an LTE network. The recipient eNB can then use the capability information when setting the configuration to be used by the UE following handover. Currently it is mandatory for a Radio Network Controller (RNC) within some originating (non E-UTRA) networks (for instance, UMTS) to provide such capabilities upon handover to an LTE network. It has been proposed to allow the RNC to not provide any LTE capabilities during handover preparation from UMTS to LTE to avoid uploading the volume of LTE capability in UMTS as this may take considerable time and delay certain operations, for instance handover to LTE.
It is also known for the E-UTRAN, specifically the eNB, to limit the volume of LTE capabilities provided by the UE by requesting that the UE only provides information regarding specific bands (requested bands). In such a case, the UE still indicates all non-Carrier Aggregation (non-CA) bands as well as all 2 downlink and 1 uplink CA band combinations, which results in a reduction in the transmitted UE E-UTRAN capability information by prioritising the CA combinations requested. The band combinations beyond 2 downlink and 1 uplink are indicated only if all of the bands of those combinations are part of the bands requested by the network. It has been agreed in principle within 3GPP that the UE shall provide the band combinations beyond 5DL (B5C) only in response to a similar kind of network request.